The present invention is related to the field of power switching devices and more particularly to voltage sensing and regulation in an automatic transfer switch system.
Automatic Transfer Switch (ATS) systems for switching between two or more power sources are well known in the art. ATS systems sense a voltage level provided by each of two or more AC power sources and automatically switch the power delivered to a load from a first AC power source to a second AC power source when the delivered voltage falls below a selected value. These systems are commonly provided, for example, between two or more AC power generators, between an AC power generator and a utility line, or in other combinations in which multiple AC power sources are used. Such systems are particularly useful in switching an appropriate alternate source of power into a system when there is a power drop or power failure situation, including blackout and brown out situations.
ATS circuits generally include a voltage sensing circuit for sensing a level of the output voltage supplied by each of the AC power sources. At an input stage, the voltage sensing circuits typically comprise transformers designed to provide a linear voltage transformation when operated in a frequency range of about 50–60 Hz, the typical operating frequencies for AC power sources. The transformer output voltage signal is evaluated to determine whether the supplied line voltage of the operating power source has fallen below an acceptable level or failed and, if appropriate, the power source is switched out and a second power source is switched in.
While prior art voltage sensing circuits are generally successful in providing an accurate sensing signal, to provide a linear transfer characteristic and prevent saturation of the core at these relatively low frequencies of operation, the transformers provided in these systems are typically large, heavy, and relatively expensive. The transformers therefore contribute significantly to the overall size and cost of the system. ATS systems which use these types of voltage sensing circuits, therefore, are also typically large and expensive. As ATS devices are used as backup systems, it is desirable to provide an ATS system which is small and inexpensive, and which can be stored easily.